Optical proximity sensors are widely used in electronic devices such as mobile phones or smartphones for detecting proximity to a human head. These sensors often use light-emitting diodes, in particular infrared light-emitting diodes, and photodetectors such as photodiodes. The sensor detects the amount of light that is reflected from the proximity target, for example the human head, as it is brought into close proximity to the sensor. Proximity detection can be used to trigger a response in the electronic device such as turning off the cell phone screen in order to conserve battery power.
One problem that occurs is optical crosstalk, since in most cases the optical proximity sensor is located behind a cover of the device, for example. This means that the photodetector detects light entering directly from the light-emitting diode or via reflection from, for example, the device cover. A common approach to reduce optical crosstalk is to design the optical arrangement such that the critical point lies outside of the device cover surface. The critical point characterizes the distance from the optical sensor below which no proximity detection is possible. Usually, the critical point is distributed in space and not an infinitesimally small point. As a consequence it can occur that there is a zone just outside the cover glass of the device where proximity detection is not possible. That means that target objects within this zone cannot be reliably detected by the sensor arrangement.